Radio frequency repeating systems are used as a means for increasing the communications range or for enhancing the received signal strength and thereby lowering the error rate within the communications link. The most common system includes the receiving of a signal at frequency F.sub.1, shifting it to frequency F.sub.2, amplifying the shifted frequency and retransmitting it at a higher power level. This F.sub.1 F.sub.2 system avoids interference between the received and retransmitted signals, but it occupies twice the bandwidth. A number of systems have been proposed for operation at the same input and output frequency. This invention discloses significant improvements to those F.sub.1 F.sub.1 repeating systems which use negative FM feedback. The classic use of negative FM feedback is disclosed in Enloe in an article published in the Proceedings of the IRE, January 1962, page 18, and entitled "Decreasing the Threshold in FM by Frequency Feedback". Enloe shows that negative FM feedback can result in system instability especially at gain levels required to create a useful retransmitted index of modulation. Negative feedback FM by definition produces a reduction of peak-to-peak deviation and as taught by Enloe is used for the purpose of threshold extension. The use of negative feedback in F.sub.1 F.sub.1 repeaters has resulted in poor stability and in non-equal retransmitted deviations.
Negative feedback systems, as practiced in the prior art, utilize a feedback loop in which the peak-to-peak deviation of the received IF signal is reduced. Generally this is accomplished by varying the frequency of the system oscillator so as to offset the received signal instantaneous frequency modulation at the mixer output. Such a system produces a change in the deviation at the output as compared with the deviation of the input signal, except when the gain of the feedback loop is infinite. Stated in another way, deviation at the receiver input is fixed. Deviation at the receiver IF is reduced as a function of the negative feedback loop gain until the deviation at the IF is zero when the gain is infinite. It is only when the gain is infinite and the IF deviation is zero that the transmitted deviation equals the received deviation. Since a practical system does not have infinite loop gain, the retransmitted signal will have a different deviation from the received signal. The system disclosed in accordance with this invention overcomes this problem of the prior art by using a positive feedback system, and it provides stability by providing a loop gain of less than one.
In addition, many prior art systems utilize a single antenna for both receiving and retransmitting, and isolation is provided by means of the well-known hybrid. However, hybrids never are 100% efficient and various techniques are provided for preventing the retransmitted signal from acting upon the receiver. The most common technique is to reduce retransmitting power. However, in accordance with this invention I provide the receiver IF filter with a narrow notch at its center frequency, and thus provide additional isolation in the receiver from the retransmitted signal. Moreover, because I am able to maintain a large deviation rate because of the use of a positive feedback system, the time the IF frequency is within the narrow notch is reduced. Thus, the use of the notched IF filter in combination with positive feedback from the audio to the IF permits me to increase the retransmitted power levels. The notch is able to perform its function of isolation since the retransmitted signal in the IF is a fixed nonmodulated, nondeviated signal (CW) and as such theoretically occupies zero bandwidth and therefore can be notched out by a very narrow band notch filter.